This invention pertains to the electrical power art and, more particularly, to a means for suppressing high voltage transients induced on an alternating current carrying line. Overvoltage protectors are well known in the prior art. FIG. 1 is a schematic diagram of an overvoltage protector for use in a commercial airplane. Here, the airplane's generator 10 produces at its output terminals 12, 14 an AC voltage having a nominal peak value of 162 volts at a frequency of 400 hertz. The output from the generator 10 is passed through a feeder inductance 16, here represented as a lumped inductance, to the feeder line 20. Feeder line 20, as well as generator 10, are subject to induced high voltage transients, such as may occur due to a lightning strike on the aircraft. The high induced voltages may result in damage to the aircraft's electrical equipment, illustrated as load 22. Thus, a lightning protector, indicated generally at 24, is provided between the generator's terminals 12, 14.
The lightning protector 24 includes a voltage sensing device 30 which is wired directly across the generator's terminals 12, 14 and includes internal circuitry (not shown) which senses the voltage on the line. If the peak voltage exceeds 250 volts, the voltage sensing device 30 produces a trigger signal at its outputs 32, 34. While a detailed schematic diagram of the voltage sensing device 30 is not shown herein, such circuits are well known to the prior art.
The trigger signals produced at the outputs 32, 34 of the voltage sensing device 30 are coupled via transformers 40, 42, respectively, to the gate--cathode connections of a pair of silicon controlled rectifiers 50, 52. The silicon controlled rectifiers 50, 52 are connected in parallel and in reverse polarity such that the anode of one rectifier connects to the cathode of the other. As shown, the common connection of the anode of the first silicon controlled rectifier 50 with the cathode of the second 52 connects to the second generator terminal 14.
The common connection formed by the cathode of the first SCR 50 and the anode of the second SCR 52 connects through a network, indicated generally at 60, formed by the parallel connection of a capacitor 62 and a resistor 64 to the feeder line 20.
Operation of the prior art voltage protector 24 shown in FIG. 1 may be understood as follows. During normal operation of the system, the voltage produced by the generator 10 at its output terminals 12, 14 does not rise to the threshold of the voltage sensing circuit 30 and thus the SCR's 50, 52 are biased off. Capacitor 62, thus, is normally discharged and, in effect, is connected only to the generator feeder line 20.
If the transient voltage, such as may be caused by lightning, is induced onto the generator feeder 20, it is detected by the voltage sensing device 30 once it has risen to 250 volts. A trigger voltage is then applied to the gates of both SCR's 50 and 52. One of these will be switched on depending upon whether the feeder voltage is positive or negative. That is, SCR 50 is switched on for negative feeder voltages whereas SCR 52 will switch on for positive feeder voltages. The switching on of one of the devices 50, 52 causes one end of capacitor 62 to be clamped near the potential at generator line 14, which potential is commonly airplane ground.
The subject matter of the instant invention includes the recognition of inherent limitations in the prior art design which render it incapable of accomplishing its desired purpose under all conditions. One problem with previous transient limiters of this type is that they rely on internally stored energy to supply the trigger generation circuit, which stored energy may be depleted by the operation of the circuit during occurrence of a transient. The transient limiter is then unresponsive for a certain period of time until the trigger circuit energy store is replenished. This energy store has typically been charge on a capacitor.
Another potential cause of unresponsive trigger circuitry lies in the use of transformers to couple the trigger signals to the SCR's. The production of one trigger signal may leave the transformer core flux in a state such that a core flux resetting action is necessary before a second trigger signal of equal power can be produced.
When either, or both of these causes operate to reduce the power level of the trigger signals for a brief period after a transient supression action, the transient limiter may be rendered ineffective in suppressing additional transients that occur during recovery time.